For the purpose of alleviating the driver's effort to maneuver a large-sized truck, bus or the like, an automatic transmission system capable of automatically selecting the gear position depending on the running condition of the vehicle has been proposed.
Conventional automatic transmission systems are exclusively designed for use in small-sized passenger cars.
This automatic transmission system has generally such a structure that a fluid coupling such as a hydraulic torque converter is interposed between an engine and a planetary gear type transmission controlled by hydraulic pressure, and the gear position of the planetary gear type transmission is changed over by gear position change-over means.
An important matter to be taken into consideration for the development of an automatic transmission system suitable for application to a large-sized truck or the like is that the number of production of vehicles of this kind is very small as compared to that of passenger cars. Therefore, newly designing an expensive torque converter or the like is quite disadvantageous from the aspect of cost, and it is desirable that the existing drive system including the friction clutch and transmission as well as the existing production equipment, can be directly utilized for the production without any alteration.